From the viewpoints of conserving resources and preventing environmental pollution, solvent type paints have gradually been replaced by pollution-free paints, particularly aqueous paints, and it is expected that this tendency will advance further in the future. An electrodeposition paint proposed as one of such aqueous paints has been used widely as an undercoating paint for automobiles because this paint is excellent in terms of reducing labor, reducing coating expenses and preventing environmental pollution. However, the corrosion resistance of coatings formed from anionic electrodeposition paints now available is not satisfactory. For this anionic electrodeposition paint, a polycarboxylic acid resin comprising a drying oil, an alkyd resin, a polybutadiene resin, an epoxy ester resin or an acrylic resin is used as the resin component. This resin is ordinarily neutralized by a basic compound such as an organic amine to render the resin water-soluble or water-dispersible. One of the reasons for this poor corrosion resistance of a coating of such an anionic electrodeposition paint is considered to be the presence of carboxyl groups in the cured coating. Recently, a cationic electrodeposition paint has attracted attention as a paint for eliminating the defect of the insufficient corrosion resistance involved in the anionic electrodeposition paint. A polyamino resin comprising an epoxy resin, a urethane resin, an acrylic resin, a polybutadiene resin or an alkyd resin is used as the resin component of this cationic electrodeposition paint. This resin is ordinarily neutralized by an organic acid to render the resin water-soluble or water-dispersible. It is said that a coating having a high corrosion resistance can be obtained with such polyamino resins since amino groups in the coating act as the corrosion-inhibiting agent.
Corrosion of coatings formed on automobiles, to which the above-mentioned conventional electrodeposition paints are mainly applied, is roughly divided into the following two types:
(1) So-called scab corrosion which advances from a portion of the coating damaged by an external cause (for example, impact by a stone or the like). PA1 (2) So-called perforation which advances from unsatisfactorily chemically treated portion or an incompletely coated portion.
Ordinarily, a coating of a cationic electrodeposition paint exhibits effective anti-corrosive action even on an insufficiently chemically treated steel plate. However, susceptibility to damage (1) by an external cause greatly differs depending on the kind of resin. Among the above-mentioned resins having amino groups, a coating of the urethane resin type exhibits excellent properties as flexibility, nearly complete freedom from damage by external causes and, ordinarily, superiority over coatings of other resins with respect to the balance among other properties. Such cationic urethane resins are disclosed, for example, in Japanese Patent Publications No. 36958/73, No. 17234/75, No. 24982/75, No. 2491/76, No. 38317/78, No. 2894/78 and No. 2895/78 and Japanese Patent Application Laid-Open Specifications No. 96696/73, No. 99297/73 and No. 101430/73. However, these cationic urethane resins are still defective in coating properties such as corrosion resistance and water resistance.
Furthermore, it is known to incorporate a phenolic resin in an epoxy resin type or acrylic resin type cationic coating composition. For example, there are known (1) a composition comprising a polyepoxide solubilized by a quaternary onium salt and an amine/aldehyde condensate and/or unsaturated methylol phenyl ether (see the specifications of U.S. Pat. Nos. 3,937,679 and 4,038,166), (2) an aqueous lacquer comprising an addition polymer, or a polycondensation polymer, for example a nitrogen-containing basic resin of the acrylic or epoxy ester type, and if desired, an aminoplast and/or phenolplast, in which the phenolplast is partially or completely etherified by a lower alcohol (see Japanese Patent Publication No. 770/77), (3) a composition comprising an acrylic resin and, if desired, a water-soluble phenol-formaldehyde resin (see the specifications of U.S. Pat. Nos. 3,446,723, 3,455,806 and 3,454,482), (4) a coating composition comprising an epoxy resin and, if desired, an amino resin or phenolic resin etherified by an alcohol (see the specification of British Pat. No. 1,307,585), and (5) a composition comprising an epoxy resin, an epoxy ester resin or a reaction product of an epoxy resin or epoxy ester resin with trishydroxylmethylphenol allyl ether or the like (see Japanese Patent Application Laid-Open Specification No. 134897/78).
Furthermore, there is known a composition comprising a cationic water-dispersible resin, a hydrocarbon resin and/or modified hydrocarbon resin and, if desired, an amine-formaldehyde resin or a phenol-formaldehyde resin, for example, a composition comprising an acrylic, epoxy or urethane type cationic water-dispersible resin and a polydiene type hydrocarbon resin (see West German Patent Application Laid-Open Specification No. 2,636,797).
In these known compositions, the epoxy resin is defective in that the flexibility of the coating is insufficient and in that a decline in corrosion resistance is observed after performing impact resistance tests, while the acrylic resin or epoxy ester resin is defective in that the corrosion resistance and water resistance are insufficient. Furthermore, a composition comprising a water-soluble phenolic resin in combination with the resin having amino groups is similarly insufficient in corrosion resistance and water resistance, and a composition comprising an etherified phenolic resin incorporated in or bonded to the resin is defective in that properties of a coating cured at a low temperature (lower than 180.degree. C.) are insufficient and the corrosion resistance and water resistance are poor. In a composition comprising a polybutadiene resin and a phenolic resin in combination with the resin having amino groups, the phenolic resin acts as an anti-oxidant while the polybutadiene resin is an oxidative polymerization type resin. Hence, this composition is defective in that a satisfactory cured coating cannot be obtained.
We have conducted research with a view to developing a coating composition comprising a urethane resin in which the corrosion resistance and water resistance are highly improved while retaining excellent properties inherent to a coating of the urethane resin type, such as high flexibility. We have now completed the present invention based on the results of this research.
More specifically, we have found that when a water-insoluble resol-type phenolic resin is used in combination with a urethane resin, the corrosion resistance and water resistance of the resulting coating can be remarkably improved without diminishing the physical properties of the coating, such as the flexibility, while improving the appearance of the coating (occurrence of popping is prevented) and attaining good low-temperature curability. The present invention has been perfected based on this finding.